Dispersion Compensating Gratings (DCGs) are known. See, for instance, Ennser et al., IEEE J. of Quantum Electronics, Vol. 34, pp. 770-778 (1998). Briefly, a DCG is a Bragg grating that is chirped over at least a part of its length, thus providing for incident light different path lengths, and therefore different delays, as a function of wavelength. This is shown schematically in FIG. 1, wherein numeral 10 refers to a single mode optical waveguide (typically conventional silica-based optical fiber), and 11 refers to a chirped Bragg grating in the fiber. Numeral 12 refers to multi-wavelength radiation propagating in the waveguide towards the grating, and 13 and 14 refer to reflected radiation. Assuming that the grating spacing decreases as a function of distance from the input end of the grating then the wavelength of radiation 13 is greater than the wavelength of radiation 14. Since radiation 14 travels a greater distance in the grating than radiation 13, the former typically has longer delay than the latter.
As is evident from the above description, a DCG provides chromatic dispersion. The sign and amount of chromatic dispersion can be chosen at will (e.g., by choice of the chirp), and the DCG can be used to compensate chromatic dispersion in a communication system.
Although very useful in principle, prior art DCGs have a disadvantageous property that so far has prevented their wide-spread use. Specifically, prior art DCGs have relatively high polarization mode dispersion (PMD) that is substantially unpredictable.
PMD occurs when one polarization of the radiation (to be referred to as "light", regardless of wavelength) at a given wavelength has a different delay in a device than the other polarization of light at the same wavelength. "Device" is to be understood broadly, to include, for instance, conventional optical fiber.
Most devices have some PMD, and fiber itself has intrinsic PMD due, inter alia, to twist during fiber drawing and/or azimuthal asymmetry of the fiber refractive index. It is generally difficult to keep the PMD of optical fiber below about 2.times.10.sup.-7.
By way of example, frequently encountered specifications for an optical device require less than 1 ps delay due to PMD. A Im section of optical fiber may have a PMD of 5 fs, substantially less than the allowed maximum. However, the same length of fiber with a grating having -1000 ps/nm dispersion in the 1.55 .mu.m wavelength region will have a PMD of over 1.5 ps, more than the permitted maximum.
The above example demonstrates that the presence of a DCG in a fiber can significantly exacerbate the intrinsic PMD of the fiber.
Twist during fiber drawing and azimuthal asymmetry are not the only PMD-generating mechanisms. Another such mechanism is UV-induced birefringence, due to non-symmetrical exposure of the optical fiber to UV during grating manufacture. UV-induced birefringence typically is about 10.sup.-6, almost double the intrinsic fiber birefringence.
The combination of uncontrolled fiber birefringence and uncontrolled UV-induced birefringence of a grating-containing device means that the PMD of the device is substantially unpredictable and cannot be easily cancelled out.
In the prior art there are attempts to deal with the PMD problem in DCGs by minimization of the birefringence in the grating. However, it is difficult to achieve birefringence less than about 10.sup.-6, and therefore difficult to achieve PMD less than about 10 ps in a DCG.
The above considerations can be quantified as follows: The polarization mode delay .tau..sub.PMD in a length L.sub.f of the fiber, with fiber birefringence B and fiber group velocity V.sub.g, is about EQU BL.sub.f /V.sub.s.
On the other hand, the polarization mode delay for light of wavelength X in a DCG wherein the penetration length before reflection is z, and wherein the dispersion is D, is about EQU B[(2z/V.sub.g)+D.lambda.].
In the latter expression, the dispersion term BD.lambda. will almost always dominate the penetration depth term 2Bz/V.sub.g, and the PMD delay of the grating will be dominated by the dispersion term.
Clearly, a technique of reducing PMD in a DCG that does not rely on minimization of the birefringence in the DCG is needed. This application discloses such a technique.